[packages] olsrd: init: cleanup an expensive/forky call

[packages] olsrd: init: cleanup an expensive/forky call

for determining all gateways in all routing tables
1) write out the ip-call, never ever abbreviate in scripts
2) avoid forks: sorting/uniq is not needed, if we only append
   yet unappended words
3) use an extra function() for this, better readable
4) check for installed ip-command

Signed-off-by: Bastian Bittorf <bittorf@bluebottle.com>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@35680 3c298f89-4303-0410-b956-a3cf2f4a3e73